AUTOMATED CRYSTAL ORIENTATION MAPPING (ACOM) - A NEW PERSPECTIVE ON THE CHARACTERIZATION OF MICROSTRUCTURE

Robert A. Schwarzer

G "Textur und Anisotropie kristalliner Stoffe" TU Clausthal, Großer Bruch 23, D-38678 Clausthal-Zellerfeld,
E-mail: schwarzer@tu-clausthal.de

Keywords: Electron diffraction, Kikuchi patterns, BKD, EBSD, electromigration

Thin layers of metals or non-metals usually have a polycrystalline structure which is characterized by size, shape, arrangement, orientation, phase, composition and residual stress of the constituting crystallites. Preferred crystal orientations and their statistical distribution in a material - the crystal "texture" - are of major scientific interest and of great importance in a wide range of industrial applications due to the dependence of many physical properties (related to the bulk, grain or grain boundary) on the crystallographic direction of testing (anisotropy). Hence monitoring of microstructure as well as crystal texture, as most sensitive indicators of the production process, is mandatory. A noticeable advancement in spatially resolved diffraction analysis has been reached recently by the development of Automated Crystal orientation Measurement (ACOM) from Backscatter Kikuchi Patterns (BKP; also named EBSP) in the SEM. A trademark for a commercial system is OIM (= Orientation Imaging Microscopy).

The main features of our ACOM system [1] are:

• An excellent pattern quality.
• Large acceptance angles of about 120°  by  100°.
      Acquisition and digitization of microstructure images in addition to diffraction patterns.
• Convenient operation under Windows® 95 / NT on a Pentium based system.
      High speed: more than 10,000 orientations per hour are measured unattendedly.
• Full control of microscope operation by the indexing software, in particular:
• Reading and writing microscope magnification, accelerating voltage, focus setting (working distance, WD), and mode of SEM operation (scanning or spot mode).
• Digital beam scan with 4,096  by 4,096 addressable image points.
• Software-controlled dynamic focus.
• Calibration of pattern center and diffraction length by the software.
• The last two features are essential for automated measurement of fine structures at low magnifications and on large areas. As a consequence of the steep specimen tilt the working distance (lens focus), screen-to-spot distance (camera length) and the pattern center vary with spot position on the specimen surface during ACOM operation. Dynamic focusing controlled by the computer has been implemented to accommodate any type of SEM. The precise knowledge of the pattern center (the footpoint from the screen to the point of incidence of the primary beam on the specimen) is vital for indexing and calculation of the reference directions.

The objectives of ACOM are the determination of crystal orientations, misorientations, grain boundary characterization, texture and stereology for a quantitative representation of microstructure of bulk surfaces on a grain-specific level. It is based on the fully automated acquisition and processing of BKP in the SEM. Orientation measurement is made at spatially specific points by a digital scan of the electron beam spot in a raster grid on the sample. The generation of a BKP requires a stationary electron probe focused on a virtually perfect volume of crystal. By inelastic scattering the primary electrons are spread out into all directions, and a fraction is backdiffracted from the interaction volume to form a Kikuchi pattern on a phosphor screen close to the surface. The pattern consists of narrow bands which can be interpreted in a first approximation by gnomonic projection of the diffracting families of lattice planes. To make allowance for strong forward scattering, the specimen surface is steeply inclined to the beam. The patterns are acquired by a high-sensitive camera, digitized and transferred to the computer. By image processing the positions and widths of the bands are extracted which yield the interplanar angles and interplanar spacings (Bragg angles) as required for indexing and orientation calculation. Spatial resolution of BKP from the bulk is presently in the range of 0.1 to 0.5  µm, depending on the beam voltage and the mean atomic number. The diffuseness of a pattern is a measure of local dislocation density. At each point of the raster grid, the x-y coordinates are recorded that describe the position, along with three Euler angles describing the local grain orientation, and the pattern quality describing the local lattice strain.

The benefits of analytical electron microscopy are thus the correlation of morphology (from the micrograph) and element composition (from EDX analysis) with crystal lattice orientation on a submicron scale. Therefore ACOM will enable a major advancement in stereological evaluation of the microstructure. The number fraction of array points assigned by measurement to a specific property (e.g. crystal orientation or lattice strain) directly stands for the area fraction of grains having this considered property. In addition all grain boundaries exceeding some tenth of a degree of misorientation are reliably identified. The correlation of spatial and orientation parameters thus allows for merging the two conventional but separated fields of microstructure characterization that are quantitative stereology and texture analysis to a more general concept of orientation stereology.

A final goal of ACOM in the SEM is a 3D reconstruction of the microstructure of the bulk by using serial sectioning in order to represent morphological as well as crystallographic features through the volume in a quantitative sense (3D orientation stereology). An interesting aspect is the connectivity of microstructure. It is suspected that it strongly influences some properties of interest such as fracture, fatigue and some forms of corrosion.

Illustrative examples of application on thin films (e.g. effect of texture on electromigration in interconnect metallization lines) and bulk materials are given. Crystal orientations, misorientations or derived entities are depicted in pseudo-colors on the grid to form crystal orientation maps or "images" of the microstructure.

1      R.A. Schwarzer: Review Paper: Automated Crystal Orientation Mapping Using a Computer-Controlled SEM. MICRON 28(1997)249-265